Thermal limiter construction and electrical switch and system utilizing the same

ABSTRACT

A thermal limiter construction having a thermally responsive device provided with a body portion and a pair of leads extending from the body portion for interconnecting the device in an electrical circuit to be controlled thereby, the device opening the electrical connection between the leads thereof when the device is sensing a certain temperature condition. A sleeve is telescoped over the body portion of the device with the sleeve having an inner insulative peripheral surface disposed adjacent the device and an outer electrical heater thereon which when energized will provide the certain temperature condition to be sensed by the device to open the electrical connection between the leads. Such thermal limiter construction can be utilized with a switch housing so that the electrical heater will only be operated when the switch thereof is in a closed position, such switch forming part of an electrical interlock control system for the oven door of a microwave oven or the like.

United States Patent 1 Plasko 1 THERMAL LIMITER CONSTRUCTION AND ELECTRICAL SWITCH AND SYSTEM UTILIZING THE SAME [75] Inventor: Emil Robert Plasko, Dayton. Ohio [73] Assignee: Micro Devices Corporation. Dayton.

Ohio

[22] Filed: June 4, 1974 [2]] Appl No.: 476,313

Related US. Application Data [62] Division of Ser. No. 344 745, March 26, 1973. Pat.

l52| US. Cl..... 219/1055 D; 99/451; 219/1055 M; 219/413 [51 Int. Cl. H0511 9/06 {58] Field of Search. 99/45l; 219/1055 R, 10.55 C. 219/1055 D, 10.55 B, 10.55 M, 413, 414,

[ Aug. 19, 1975 Primary Emmr'ncr-J. D. Miller Ass/slum l:.\uml'nerFred E. Bell Attorney. Agent, or FirmCandor, Candor &'Tassone [57] ABSTRACT A thermal limiter construction having a thermally responsive device provided with a body portion and a pair of leads extending from the body portion for interconnecting the device in an electrical circuit to be controlled thereby, the device opening the electrical connection between the leads thereof when the device is sensing a certain temperature condition. A sleeve is telescoped over the body portion of the device with the sleeve having an inner insulative peripheral surface disposed adjacent the device and an outer electrical heater thereon which when energized will provide the certain temperature condition to be sensed by the device to open the electrical connection between the leads. Such thermal limiter construction can be utilized with a switch housing so that the electrical heater will only be operated when the switch thereof is in a closed position such switch forming part of an electrical interlock control system for the oven door of a microwave oven or the like 6 Claims, 9 Drawing Figures THERMAL LIMITER CONSTRUCTION AND ELECTRICAL SWITCH AND SYSTEM UTILIZING THE SAME This is a division of application Scr. No. 344.745. filed Mar. 26. 1973. now US. Pat. No. 3.829.809.

This invention relates to an improved thermal limiter construction as well as to a switch construction in combination therewith and an electrical control system utilizing such a thermal limiter construction or the like.

It is well known from the United States patent to Plasko. US. Pat. No. 3.649.942. that a thermal limiter construction can be provided for an electrical circuit by providing a thermally responsive device to be interconnected into the electrical circuit and which will open such electrical circuit when the thermally responsive device senses a certain temperature condition. An electrical heater means can be provided and can be selectively positioned adjacent such thermally responsive device so that when the electrical heater means is energized. the same will create the certain temperature condition that will cause the thermally responsive device to open the electrical circuit.

Such thermally responsive device can comprise the thermally responsive fuse device disclosed in the US. patents to Merrill, US. Pat. Nos. 3.180.958 and 3.5 19,972. or any other thermally responsive device as desired.

The electrical heater means can be so constructed and arranged so that the same will he only energized when an adverse condition is being provided which would he harmful if the electrical circuit were permitted to continue to operate. For example. such adverse situation could be provided by a malfunctioning output of the electrical circuit or the like.

It is a feature of this invention to provide an improved thermal limiter construction of the above type.

Another feature of this invention is to provide a thermal limiter construction in combination with an electrical switch means that will operate the electrical heater means of the thermal limiter construction when the electrical switch means is in a certain operative condition thereof.

Another feature of this invention is to provide an electrical interlock control system for the door of a microwave oven or the like wherein a thermal limiter construction is utilized in such electrical control system. such thermal limiter construction comprising the thermal limiter construction of this invention or other thermal limiter construction as desired.

In particular. one embodiment of the thermal limiter construction of this invention comprises a thermally responsive device having a body portion and a pair of leads projecting from the body portion and being utilized for interconnecting the device in an electrical circuit to be controlled thereby. the thermally responsive device having means for opening the electrical connection between the leads thereof when the device is sensing a certain temperature condition. A sleeve is telescoped over the body portion of the device. the sleeve comprising an inner insulative peripheral surface adja cent the device and an electrical heater means which when energized will provide the certain temperature condition to be sensed by the device to open the electrical connection between the leads and thus open the electrical circuit. The sleeve has opposed open ends and is continuous between the ends thereof. the leads of the thermally responsive device projecting out of such open ends. The electrical heater means can comprise a conductive coating disposed on the outer pe ripheral surface of the sleeve.

Such a thermal limiter construction can he carried by a switch housing of this invention which has means for electrically connecting the switch means and the heater means in series whereby the heater means is only adapted to be energized when the switch means is in a closed condition thereof. The switch housing has a plurality of spring clips which detachably interconnect the thermal limiter construction thereto. two of such spring clips respectively being interconnected to the leads of the thermal limiter construction while another pair of such spring clips are disposed against the conductive coating on the sleeve in spaced relation to provide electrical connections thereto.

Such an electrical switch and its thermal limiter con struction can be utilized in an electrical control system of this invention for providing an electrical interlock for the door of a microwave oven that has a pair of power source lead means leading from a power source to the energy generating means for the oven. An oven door operated electrical switch is provided for this system which is disposed in one of the power source lead means. such switch being normally closed only when the door is in a closed position thereof and being normally open only when the door is in an open position thereof. A thermally responsive device is disposed in one of the power source lead means for opening its respective power source lead means when the device is sensing a certain temperature condition. Means are provided for creating the certain temperature condition when the switch abnormally remains closed and the oven door is opened to terminate the operation of the energy generating means.

When the thermally responsive device of the above electrical control system comprises the thermal limiter construction of this invention in combination with the electrical switch construction of this invention. the electrical circuit of the heater means and the electrical switch therefor is disposed across the power source leads and the switch is arranged to be only closed when the oven door is in an open position thereof and to be only opened when the oven door is in a closed position thereof.

In this manner. should the first-mentioned electrical switch abnormally remain in a closed position thereof and the oven door is moved to an open position thereof. so that electrical current would continue to be supplied to the energy generating means of the microwave oven. such opening of the door closes the switch for the heater means of the thermal limiter construction and places the same across the power source leads whereby the heater means create the aforementioned certain temperature condition that causes the thermally responsive device to open and thereby terminate the flow of current to the energy generating means.

Accordingly. it is an object of this invention to provide an improved thermal limiter construction having one or more of the novel features set forth above or hereinafter known or described.

Another object of this invention is to provide an improved electrical switch and thermal limiter construc tion arrangement having one or more of the novel features set forth above or hereinafter shown or described.

Another object of this invention is to provide an improved electrical control system for a microwave oven or the like. the control system of this invention having one or more of the novel features set forth above or hereinafter shown or described.

Other objects. uses and advantages of this invention are apparent from a reading of this description which proceeds with reference to the accompanying drawings forming a part thereof and wherein:

FIGv I is a perspective view of an improved thermal limiter construction of this invention.

FIG. 2 is an exploded perspective view of the parts of the thermal limiter construction of FIG. I.

FIG. 3 is an enlarged. fragmentary. cross-sectional view taken on line 3-3 of FIG. 2.

FIG. 4 is a side view of the improved electrical switch and thermal limiter construction arrangement of this invention.

FIG. 5 is a top view of the switch arrangement illustrated in FIG. 4.

FIG. 6 is an end view of the switch arrangement illustrated in FIG. 4.

FIG. 7 is a cross-sectional view taken on the line 77 of FIG. 5 and illustrating the electrical switch in a closed condition thereof.

FIG. 8 is a view similar to FIG. 7 and illustrates the switch construction in an open position thereof.

FIG. 9 is a schematic view illustrating the electrical control system of this invention.

While the various features of this invention are hereinafter described and illustrated as providing a thermal limiter construction adapted to be utilized with a particular electrical switch construction of this invention for use in an electrical control system of this invention for a microwave oven. it is to be understood that the thermal limiter construction by itself, as well as in combination with the electrical switch construction of this invention. can be utilized for other purposes. as de sired.

Therefore. this invention is not to be limited to only the embodiment illustrated in the drawings. because the drawings are merely utilized to illustrate one of the wide variety of uses of this invention.

Referring now to FIGS. 1-3. the improved thermal limiter construction of this invention is generally indicated by the reference numeral and comprises a thermally responsive device that is generally indicated by the reference numeral 2| and. in this particular embodiment of this invention. comprises one of the thermally responsive fuses of the type set forth in the aforementioned U.S. patents to Merrill. Such thermally responsive device 21 comprises a generally cylindrical body portion 22 having leads 23 and 24 extending from opposed ends 25 and 26 of the body portion 22 with the device 21 having means therein for normally completing an electrical circuit between the lead means 23 and 24 until the device 22 senses a certain temperature condition. When the device 21 senses such temperature condition. a fuseablc pellet within the body por' tion 22 collapses and causes an electrical switching operation within the body portion 22 that opens the electrical circuit between the leads 23 and 24. Thus. when the device 2] is utilized in an electrical circuit. such opening of the switch means in the device 21 terminates the flow of current in the electrical circuit being protected thereby and since the device 2] cannot be reset, such device 2I, when blown. must be replaced before the electrical circuit can again be utilized.

A pair of metallic cup-shaped ferrules 27 are pro vided and each has a closed end 28 and a substantially cylindrical side wall means 29 defining an open end 30 remote from the closed end 28 thereof. The closed end 28 has a centrally located and outwardly projecting tubular portion 31 that has an opening passing therethrough and also passing through the end wall 28 so that the end of the respective lead 23 or 24 be inserted through the open end 30 of the particular ferrule 27 and be received in the tubular extension 31 whereby the cylindrical side wall 29 of the ferrule 27 will be fully centered on the particular lead 23 or 24. Thereafter. the assembled ferrule 27 can be secured to its respective lead 23 or 24 by solder. staking or the like.

However. before at least one of the ferrules 27 is secured to the lead 23 or 24 of the thermally responsive device 21. a sleeve 32 of this invention is telescoped over the body portion 22 of the thermally responsive device 21 in the manner illustrated in FIG. I whereby the leads 23 and 24 project out of opposed open ends 33 and 34 of the sleeve 32 and can then have the ferrules 27 subsequently secured thereon in the manner previously described.

In the particular embodiment of the thermal limiter construction 20 of this invention. the diameter of each ferrule 27 is substantially the same as the diameter of the sleeve 32 whereby once the ferrules 27 are secured in place on the leads 23 and 24, the sleeve 32 cannot be removed from its telescoped relation over the thermally responsive device 2I whereby such sleeve 32 need not be fastened to the device 2l. However. if de sired. the sleeve 32 can be centered in the proper position as illustrated in FIG. I for the purpose hereinafter described and can be secured in such telescoped relation by any suitable adhesive disposed between the sleeve 32 and the body portion 22 of the device 21. as desired.

The sleeve 32 comprises a glass tube 35 as illustrated in FIG. 3 and is preferably formed of a glass material having a relatively low coefficient of thermal expansion. Since the glass tube 35 is electrically insulative, it can be seen that the internal peripheral surface 36 of the sleeve 32 is disposed adjacent the outer cylindrical surface 37 of the body portion 22 of the device 21 and since the body portion 22 has the outer peripheral surface 37 thereof formed of conductive material. the glass tube 35 provides electrical insulation about the same.

The outer peripheral surface 38 of the glass tube 35 has a conductive coating 39 disposed thereon with the conductive coating 39 providing an electrical heater means for the device 2l as will be apparent hereinafter. While the conductive coating 39 can be formed of any suitable material which will cause the same to act as a heater means when an electrical current is passed therethrough in the manner hereinafter described. such coating 39 can comprise a tin oxide coatingv In order to enhance the electrical connection to the conductive coating 39. hands 40 of silver paint can be provided at the opposed ends 33 and 34 of the sleeve 32 on top of the coating 39 in the manner illustrated in FIGS. 1 and 2 so that when electrical connection is made respectively to the bands 40. electrical current is adapted to flow from one band 40 through the coating 39 to the other band 40 and such flow of current through the coating 39 causes the coating 39 to heat up and provide the certain temperature condition that the thermally responsive device 21 must sense before the device 21 will open and terminate the electrical connection between the leads 23 and 24 thereof.

For exam plc. it has been found that when the thermal limiter construction has been formed in the above manner and is utilized in the electrical control system of this invention for providing the electrical interlock means for the oven door of a microwave oven in the manner illustrated in FIG. 9. only about four seconds are required for a flow of current through the coating 39 of the sleeve 32 to cause the device 2] to blow and thereby open the circuit between the leads 23 and 24 because of the thermal delay provided by the glass sleeve 35 and the thermal delay provided by the pellet within the body portion 22 of the thermally responsive device 21.

This time delay in the opening of the device 21 is particularly desirable to prevent a false opening of the electrical circuit as will be apparent hereinafter.

The thermal limiter construction 20 previously described is adapted to be utilized in combination with an electrical switch housing of this invention that is generally indicated by the reference numeral 41 in FIGS. 4-8

to form an electrical switch arrangement of this invention that is generally indicated by the reference numeral 42.

The housing 4] is formed from a cup shaped body member 43 having an open end 44 adapted to be closed by an end closure 45 as illustrated in FIG. 7 to define a chamber 46 therein. An electrical Switchblade 47 is disposed in the chamber 46 and is reversely bent to define a pair of spring legs 48 and 49 respectively having contact ends 5" and 5] adapted to be spring pressed by the natural resilience of the springblade 47 into electrical contact with fixed contact means 52 and 53 whereby the blade 47 and fixed contact means 52 anad 53 define an electrical switch 41' within the housing means 41.

The fixed contact means 52 comprises a rivet-like member 83 electrically interconnected to a substantially U-shapcd conductive spring clip 54 having a pair of opposed and suitably shaped spring-clip arms 55 and 56 as illustrated in FIG. 5 for clamping against opposed sides of one end of the conductive band of the sleeve 32 of the thermal limiter construction 20 when the same is assembled to the housing 4] by having the opposed ferrules 27 thereof respectively received in end conductive spring clips 57 and 58 and the other conductivc band 40 of the sleeve 32 received in a fourth conductive spring clip 59 as illustrated.

Thus. it can be seen that the thermal limiter construction 20 of this invention is adapted to be snapped into the spring clips 54. 57, 58 and 59 of the housing 4] and since the spring clips 54. 57. 58 and 59 are formed of conductive material. the ferrules 27 for the leads 23 and 24 are respectively electrically interconnected to the spring clips 57 and 58 while the conductive bands 40 of the sleeve 32 for the heater means 39 are rcspec tivcly electrically interconnected to the spring clips 54 and 59. Since the spring clip 54 is electrically interconnectcd to the rivet means 83 and. thus. to the fixed contact 52 inside the switch housing 4|. the spring clip 54 is electrically interconnected to the switchbladc 47 and if the switchblade 47 has its contact end 5] in contact with the fixed contact 53. the left-hand conductivc band 40 of the heater means 39 has one end thereof electrically interconnected to the fixed contact 53. Thus. the electrical switch 4] and heater means 39 are disposed in series for a purpose hereinafter described.

The fixed contact 53 of the switch 4| comprises part of a terminal 60 that projects outwardly from the housing 4] in the manner illustrated in FIGS. 4, 5 and 6.

The spring clips 57 and 58 are also interconnected respectively to outwardly directed terminals 61 and 62 of the housing 41 with the terminal 62 also being electrically interconnected to the spring clip 59 as illustrated in FIG. 7.

A switch actuator, such as an axially movable plunger 63, projects out through an opening 64 in the cover member 45 of the switch arrangement 42 and is prevented from extending beyond the position illustrated in FIG. 7 by a large disc-like part 65 abutting against the inside surface 66 of the cover 45 outboard of the opening 64. A reduced projection 67 on the plunger 63 extends beyond the disc 65 and passes through a suitable opening 68 formed in the leg 49 of the switchblade 47 as illustrated.

When the plunger 63 is moved inwardly from the position illustrated in FIG. 7 to the position illustrated in FIG. 8, it can be seen that the disc-like portion 65 of the plunger 63 engages against the leg 49 of the switchblade 47 and moves the same upwardly to thereby move the end 51 of the leg 49 out of electrical contact with the fixed contact 53 and thereby open the electrical circuit leading to the left side of the heating means 39 for the thermally responsive device 21. In order to prevent overtravel of the plunger 63 from adversely affceting the switchblade 47. the end 69 of the extension 67 for the plunger 63 is adapted to abut against the end 50 of the leg 48 of the switchblade 47 in the manner illustrated in FIG. 8 to limit the amount of inward movement of the plunger 63.

When the plunger 63 is released from the position illustrated in FIG. 8. the natural resilience of the springblade 47 returns to the springbladc 47 and plunger 63 to the condition illustrated in FIG. 7 whereby the end 51 of the leg 49 of the blade 47 is again placed in electrical contact with the fixed contact means 53 whereby the switch 41' is again in its closed condition.

As previously stated, the switch arrangement 42 can be utilized for any desired purpose. However, one feature of this invention is to provide an improved electrical interlock control system for a microwave oven or the like and the electrical switch arrangement 42 is particularly adapted to be utilized in such electrical control system of this invention which will now be described.

As illustrated in FIG. 9. the electrical control system of this invention is generally indicated by the reference numeral 70 and is utilized for controlling an energy producing or generating means 7] for a microwave oven 72 that has a conventional door 73 for opening and closing the cooking chamber of the same.

The electrical control system 70 comprises a pair of power source lead means 74 and 75 adapted to be respectively plugged into an electrical power source 76 by a conventional plug 77 in a manner well known in the art so that electrical current is adapted to be supplied by the power source leads 74 and 75 to the energy generating means 7] for the oven 72 in a manner well known in the art.

An electrical switch 78 is disposed in the power source lead means 74 and is suitably interconnected by tying means 79 to the oven door 73, such as by having a plunger or the like thereof controlled by the door 77 whereby the switch 78 is normally disposed in its closed position only when the oven door 73 is in its closed position and the switch 78 is normally disposed in its open position only when the oven door 73 is moved to an open position thereof. Thus. if the switch 78 is operating normally. electrical current can only be supplied to the energy generating means 71 if the oven door 73 is in its closed position as the switch 78 will open upon opening of the door 73.

However. it has been found that there are adverse times where the switch 78 abnormally remains closed when the oven door 73 is subsequently opened following a cooking operation. such as by having the contacts of the switch fried and thereby welded together, so that when a person opens the oven door 73, the switch 78 abnormally remains closed and thereby permits the energy generating means 71 to continue to operate. Thus. harmful microwaves can pass out of the opened oven door.

Therefore. in order to prevent such an adverse situation. the electrical control system 70 of this invention is provided with the switching arrangement 42 of this invention. the switching arrangement being schematically illustrated in FIG. 9 whereby it can be seen that the thermally responsive device 21 is disposed in the power source lead 74 by having the terminals 61 and 62 of the housing 41 respectively electrically interconnected in the power source lead 74 intermediate the first mentioned switch 78 and the energy generating means 71. The terminal 60 for the electrical switch 41' of the heater means 39 is electrically interconnected to a branch lead 80 that is electrically interconnected to the other power source lead 75 whereby an electrical circuit can be provided between the power source leads 75 and 74 through the branch lead means 80. electrical switch 41' and heater means 39 should the switch means 41 be in its closed position.

However. the plunger 63 of the switch means 41' is so constructed and arranged that the same is adapted to be moved to its switch opening position as illustrated in FIG. 8 only when the oven door 73 is in its closed position and is permitted to move to its closed position as illustrated in FIG. 7 only when the oven door 73 is moved to its open position.

The operation of the electrical control system 70 utilizing the electrical switch arrangement 42 of this invention will now be described.

With the door 73 of the microwave oven 72 in the open position as illustrated in FIG. 9. the energy generating means 71 for the oven 72 cannot be supplied eIcC trical current from the power source 76 because the switch 78 in the lead 74 is normally in the open condition as illustrated in FIG. 9 even though the opening of the door 73 has caused the plunger 63 of the switch arrangement 42 of this invention to assume the position illustrated in FIG. 7 where the heater means 39 is placed across the power source leads 74 and 75. However. since the switch 78 is open in the power line 74 upstream from the heater means 39. no current flows through the heater means 39.

When the housewife or the like places an item in the oven 72 to be cooked and subsequently closes the door 73, the energy generating means 71 can now be ener- (ill gized because the closing of the door 73 closes the switch 78 and opens the switch means 41' by acting against the plunger 63 and moving the same from the position illustrated in FIG. 7 to the position illustrated in FIG. 8 so that no current can flow through the heater means 39 as the current is flowing through the power lines 74 and to the energy generating means 71 to operate the same in a conventional manner. It can be seen that the current in the power line 74 flows through the thermally responsive device 21 as the same does not open until it senses the previously described certain temperature condition that can only be produced by the heater means 39 being energized.

However. if during the cooking of the food in the oven 72 by the energy generating means 71 an abnormality should occur and the switch 78 remain in its closed position even though the oven door 73 is subsequently opened so that the energy generating means 71 would continue to operate. the opening of the door 73 causes the closing of the switch 41' so that the heating means 39 is now placed across the power source leads 74 and 75 in parallel with the energy generating means 71 and since current is now flowing through the power lines 74 and 75, as the malfunctioning switch 78 is in its closed position. current flows through the heating means 39 and causes the thermally responsive device 21 to open and thereby terminate the flow of current to the energy generating means 71.

If the thermally responsive device 21 when opened in the manner previously described is of the type that must be completely replaced before the line 74 can be again utilized. it can be seen that the microwave oven 72 cannot be operated again until a new device 21 is utilized and such device 21 will again blow if the switch 78 isnt first repaired so that the same will normally open upon the opening of the oven door 73.

Once the device 21 has opened the power source line 74 in the manner previously described, current flow through the heating means 39 is terminated because the heating means 39 is interconnected to the power source line 74 intermediate the thermally responsive device 21 and the energy generating means 71.

It is desired that the switch means 78 and 41' be so constructed and arranged and be operated by the oven door 73 in such a manner that the switch means 78 will open before the switch means 41' closes upon the opening of the oven door 73 and that the switch means 41' will open before the switch means 78 closes upon the closing of the oven door 73. Such a sequence of switch opening and closing would insure that the thermally responsive device 21 would not be inadvertently blown.

However. because there is a small time delay in the opening of the thermally responsive device 21 upon the initial energizing of the heater means 39. such sequence in the opening and closing of the switches 78 and 41' need not be held within a close tolerance relationship.

Therefore. it can be seen that this invention not only provides an improved thermal limiter construction and an electrical switch arrangement therewith. but also this invention provides an improved electrical interlock system for a microwave oven door or the like.

While the forms of the invention now preferred have been illustrated and described as required by the patent statute. it is to be understood that other forms can be utilized and still come within the scope of the appended claims.

What is claimed is:

I. In an electrical control system for a micro-wave oven that has a pair of power source lead means leading from a power source to the energy generating means for the oven, the improvement comprising an oven door operated electrical switch in one of said power source lead means that is normally closed only when said door is in a closed position thereof and is normally open only when said door is in an open position thereof, a thermally responsive device in one of said power source lead means for opening its respective power source lead means when said device is sensing a certain temperature condition. and means for creating said certain temperature condition when said switch abnormally remains closed and said oven door is opened to terminate the operation of said energy generating means, said means for creating said certain temperature condition comprising a branch lead means for electrically interconnecting said two power source lead means together, said branch lead means interconnecting to the power source lead means that has said switch therein at a point intermediate said switch and said energy generating means, said branch lead means having an electrically operated heater means disposed therein adjacent said device for creating said certain temperature condition sensed by said device when current flows through said branch lead means. and another electrical switch means disposed in said branch lcad means said other electrical switch means normally being open only when said door is in a closed position thereof and being normally closed only when said door is in an open condition thereof whereby said heater means is only energized if the first named switch is abnormally closed and said door is in an open condition thereof, said other switch means carrying said device and said heater means, said heater means and said device being readily replaceablc together as a single unit.

2. In a control system as set forth in claim 1, said device being disposed in the same power source lead means as said first-named switch.

3. [n a control system as set forth in claim 2, said device being disposed in the respective power source lead means intermediate said first-named switch and said energy generating means.

4. In a control system as set forth in claim 3, said branch lead means interconnecting to the power source lead means that has said device therein intermediate said device and said energy generating means.

5. In a control system as set forth in claim 1, said device being readily replaceable from said unit.

6. In a control system as set forth in claim I, said firstnamcd switch normally opening before said other switch closes as said door is being opened and said other switch normally opening before said first-named switch closes as said door is closing. 

1. In an electrical control system for a micro-wave oven that has a pair of power source lead means leading from a power source to the energy generating means for the oven, the improvement comprising an oven door operated electrical switch in one of said power source lead means that is normally closed only when said door is in a closed position thereof and is normally open only when said door is in an open position thereof, a thermally responsive device in one of said power source lead means for opening its respective power source lead means when said device is sensing a certain temperature condition, and means for creating said certain temperature condition when said switch abnormally remains closed and said oven door is opened to terminate the operation of said energy generating means, said means for creating said certain temperature condition comprising a branch lead means for electrically interconnecting said two power source lead means together, said branch lead means interconnecting to the power source lead means that has said switch therein at a point intermediate said switch and said energy generating means, said branch lead means having an electrically operated heater means disposed therein adjacent said device for creating said certain temperature condition sensed by said device when current flows through said branch lead means, and another electrical switch means disposed in said branch lead means, said other electrical switch means normally being open only when said door is in a closed position thereof and being normally closed only when said door is in an open condition thereof whereby said heater means is only energized if the first named switch is abnormally closed and said door is in an open condition thereof, said other switch means carrying said device and said heater means, said heater means and said device being readily replaceable together as a single unit.
 2. In a control system as set forth in claim 1, said device being disposed in the same power source lead means as said first-named switch.
 3. In a control system as set forth in claim 2, said device being disposed in the respective power source lead means intermediate said first-named switch and said energy generating means.
 4. In a control system as set forth in claim 3, said branch lead means interconnecting to the power source lead means that has said device therein intermediate said device and said energy generating means.
 5. In a control system as set forth in claim 1, said device being readily replaceable from said unit.
 6. In a control system as set forth in claim 1, said first-named switch normally opening before said other switch closes as said door is being opened and said other switch normally opening before said first-named switch closes as said door is closing. 